Prostaglandin I.sub.2 (PGI.sub.2, prostacyclin) is a compound discovered in 1976 by J. R. Vane et al., which has called attention as a substance exerting potent platelet aggregation--inhibiting and gastric acid secretion--inhibiting activities and potent peripheral vasodilative activities after having been biosynthesized from archidonic acid via an endo peroxide (PGH.sub.2 or PGG.sub.2) at the vascular wall [C & E N, Dec. 20, 1976, p17. and S. Moncada, R. Gryglewaki, S. Bunting, J. R. Vane, Nature, 263, 633 (1976)]. ##STR1##
However, PGI.sub.2 which has an exo-enol structure is extremely unstable even in neutral aqueous solution and is readily subjected to conversion to 6-oxo PGF.sub.1 which has almost no physiological activities. Such instability of is a big obstacle to its use as a drug. PGI.sub.2 is also metabolized quickly in vivo and disadvantageously shows only short duration of physiological activities in vivo.
A tremendous amount of research has been made on various derivatives for the purpose of improving the chemical stability and duration of activities in vivo of PGI.sub.2.
The inventers resolved the problems of such chemical instabilities in PGI.sub.2 by inventing novel derivatives which have a cyclopenta[b]benzofuran ring containing a phenol ring in place of unstable exo-enol structure therein and filed a series of patent applications (See, Japanese patent application Nos. 36477/81, 32277/82, 14427/82, 124778/83, 134787/84, and 265279/87).
As the result of further study, it has now been found that the novel PGI.sub.2 derivatives having the general formula [I] have strong pharmacological activities and in vivo excellent stability.
An object of the present invention is to provide novel PGI.sub.2 derivatives which are excellently stable and potent in vivo.